Urea cycle disorders have emerged as potentially good candidates for the development of gene therapy. Ornithine transcarbamylase deficiency (OTC) has been the primary focus of preclinical studies. The rationale for choosing this disorder includes its frequency, its severity and resistance to treatment, and the presence of mouse models. Because the urea cycle is principally located in the liver, gene therapy directed at the liver has the potential to correct most of the metabolic derangements. The recent success of liver transplantation in these disorders further supports the development of liver directed gene therapy. In vivo gene therapy using adenoviral vectors is attractive for several reasons, including the fact that the virus can be administered by intravenous injection, the high level of expression observed after a single injection, and the rapidity of that expression. Studies of transgene expression in the mouse models of OTC deficiency are encouraging but have also provided evidence that the immune system may be involved in mediating two limiting aspects of this technology, transgenic expression and inflammation. Although deletions in adenoviral early genes limit adenoviral late gene expression and subsequent viral replication, there is evidence that late gene expression still occurs. We have developed a series of adenoviral vectors containing the mouse OTC cDNA. When used in OTC deficient sparse fur mice, we have found that the recombinant adenoviruses deleted in E1 and with a temperature sensitive mutation in E2 have successfully corrected the OTC deficiency and associated metabolic derangements for up to three months. We have also shown that immunosuppression with anti-CD4 at the time of gene transfer permits repeated subsequent injections such that correction can be extended for up to nine months. Finally, we demonstrated that pretreatment as little as 24 hours before a potentially fatal ammonium chloride load (10mmol/kg) prevented most clinical and biochemical abnormalities from occurring. This suggests rapid transgenic expression. Based on this data, and preclinical safety studies, we are currently beginning a Phase I trial of gene transfer in adults who are a symptomatic carriers or have partial OTC deficiency. The development of an approach to gene therapy that safely, stably and effectively transduces gene expression holds the promise of significantly improving the treatment of inborn errors of urea synthesis.